The present invention relates to a perfluoro compound separation recovery method to separate and recover a perfluoro compound with a membrane from diluted mixed gas in which the perfluoro compound is diluted, and a perfluoro compound separation recovery system therefor.
Perfluoro compound (simply mentioned as xe2x80x9cPFCxe2x80x9d hereinafter) gas such as CF4, C2F6, C3F8, C4F8, C4F10, CHF3, SF6 and NF3 has been conventionally diluted with a large amount of N2 gas or the like for use such as in etching, cleaning a chamber and the like in the manufacturing process of semiconductors. Also, in the field of electrical machines, SF6 is similarly used as insulating gas.
However, exhaust gas (diluted mixed gas) contains unreacted (undecomposed) PFC gas after use, causing such environmental problems as ozone layer depletion when the gas is released into the atmosphere.
Thus, PFC gas in exhaust gas has been conventionally recovered. A PFC separation recovery system shown in FIG. 8 is used to recover the gas. In the order of exhaust gas treatment, a pre-treatnent unit 1, a blower 2, a first tank 3, a compressor 4, a second tank 5, a heat exchanger 6, a PFC separation unit 51 and a PFC recovery unit 28 are connected in series in the PFC separation recovery system.
More specifically, a first membrane 7, a second membrane 8 and a third membrane 9 ate arranged in series in the PFC separation unit 51. Each membrane 7, 8, 9 has a tendency to permeate more diluent gas such as N2 gas than PFC gas. The materials of each membrane 7, 8, 9 include polysulfone, polyetherimide, polypropylene, cellulose acetate, polymethylpentane, amorphous copolymer having 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxysol as a radical, polyvinyltrimethylsilane, polyimide, polyamide, polyaramid or ethyl cellulose polymer, and the like. The materials mentioned above are formed in a hollow fiber, spiral, roll or flat sheet shape.
Moreover, spaces in the front and rear of each membrane 7, 8, 9 are a permeation side and a non-perineation side, respectively. The permeation side of the first membrane 7 is linked to the atmosphere through a needle valve 52, releasing permeated gas into the atmosphere. The non-permeation side of the first membrane 7 is connected to the second membrane 8. Also, the permeation side of the second membrane 8 is connected to return the gas to the first tank 3, and the non-permeation side of the second membrane 8 is connected to the third membrane 9. Furthermore, the permeation side of the third membrane 9 is connected to return the gas to the first tank 3, and the non-permeation side of the third membrane 9 is connected to the PFC recovery unit 28 through a first control valve 10.
Furthermore, a concentration analyzer 53 is provided to sample a portion of a third non-permeable gas flow between the first control valve 10 and the PFC recovery unit 28 so as to measure the concentration of PFC gas. A control unit 53a is also provided to control the opening of the first control valve 10 in response to the output of the concentration analyzer 53. In other words, when the concentration analyzer 53 takes in a portion of the third non-permeable gas flow and it is determined that the concentration of PFC gas is lower than a predetermined level, the control unit 53a reduces the opening of the first control valve 10, thus increasing a third permeable gas flow. Accordingly, diluent gas that was flowing to the non-permeation side flows to the permeation side, and recovery concentration (concentration of recovered PFC gas) improves. Thus, PFC gas of high concentration may be recovered by controlling the flow amount of the third non-permeable gas flow. The portion of the third non-permeable gas flow that was taken into the concentration analyzer 53 returns to the first tank 3 after the measurement of concentration.
A third tank 11, a booster pump 12 and a recovery container 13 are arranged in series in the PFC recovery unit 28.
The separation recovery of PFC gas is carried out as follows by the PFC separation recovery system. In other words, exhaust gas is first passed through the pre-treatment unit 1, removing an acid-based portion, a particle portion and the like which provide negative effects on the compressor 4 and each membrane 7, 8, 9, with a wet or dry scrubber, a filter and the like. Subsequently, the treated gas flow is stored in the first tank 3 by the blower 2. Then, the compressor 4 raises the pressure of a gas flow from the first tank 3 to store it in the second tank 5. The heat exchanger 6 raises the temperature of exhaust gas stored in the second tank 5. Then, the gas contacts the first membrane 7, thus separating the gas into a first permeable gas flow rich in diluent gas and a first non-permeable gas flow rich in PFC gas. Additionally, the first permeable gas flow is released into the atmosphere by opening and closing the needle valve 52. At the same time, the first non-permeable gas flow contacts the second membrane 8, thereby further separating the gas into a second permeable gas flow rich in diluent gas and a second non-permeable gas flow rich in PFC gas. Then, the second permeable gas flow is returned to the first tank 3. At the same time, the second non-permeable gas flow contacts the third membrane 9, thus separating the gas into a third permeable gas flow having diluent gas at high concentration and a third non-permeable gas flow having PFC gas at high concentration. Then, the third permeable gas flow is returned to the first tank 3. At the same time, the third non-permeable gas flow is stored into the third tank 11 through the first control valve 10. Subsequently, a gas flow from the third tank 11 is stored in the recovery container 13 by a booster pump 12. PFC gas can be separated and recovered as noted above.
In the above-noted PFC separation recovery method, pressure (internal pressure) is controlled at a constant level at the upstream side of the blower 2 and the upstream side (first tank 3) and the downstream side (second tank 5) of the compressor 4 for its stable operation.
In other words, a first pressure sensor 14 is provided before a section where exhaust gas flows into the PFC separation recovery system (the upstream side of the pre-treatment unit 1) in order to indicate the pressure of exhaust gas. Also, an inverter 2a is provided to the blower 2 to control its operation. At the same time, the inverter 2a is provided with a control unit 14a to send signals to itself in response to the output of the first pressure sensor 14. The inverter 2a starts the blower 2 to provide the output of the first pressure sensor 14 at a predetermined fixed level. Pressure at the upstream side of the blower 2 is controlled at a constant level thereby. The first pressure sensor 14 indicates strength values for sucking exhaust gas. The flow of exhaust gas is made stable and smooth by controlling the values so as to be constant.
As described above, gas flows into the first tank 3 after the blower 2, the permeation side of the second membrane 8, the permeation side of the third Membrane 9 and the concentration analyzer 53, and then flows out to the compressor 4. Additionally, there provided are a first line 15 to take in outside air, and a second line 16 returning from the second tank 5. At the same time, a second pressure sensor 17 is provided to indicate the internal pressure of the first tank 3. Furthermore, a second control valve 18 is Provided at the first line 15, and a control unit 17a is provided to the second control valve 18 to control its opening in response to the output of the second pressure sensor 17. The second tank 5 is also provided with a third pressure sensor 19 to show its internal pressure. Furthermore, the second line 16 is provided with a third control valve 20, and a control unit 19a is provided to the third control valve 20 to control its opening in response to the output of the third pressure sensor 19. The internal pressure of the first tank 3 is controlled at a constant level by controlling the second control valve 18 of the first line 15. The internal pressure of the second tank 5 is controlled at a constant level by controlling the third control valve 20 of the second line 16. In other words, when the amount of gas flowing from the blower 2 into the first tank 3 decreases, the second control valve 18 is gradually opened to supplement a deficient amount of gas with outside air, thus controlling the internal pressure of the first tank 3 at a constant level. This control is achieved by controlling the internal pressure of the first tank 3 (set pressure of the second pressure sensor 17) at a negative level.
However, in the conventional art mentioned above, the expensive concentration analyzer 53 is necessary to control the first control valve 10. It is also necessary to perform fine concentration adjustment to raise recovery concentration to a limit so as to provide a recovery rate (a rate between the amount of recovered PFC gas and the amount of exhaust gas) at a predetermined level or higher. Thus, the concentration analyzer 53 of extremely high precision becomes necessary. Moreover, since the concentration analyzer 53 requires time for concentration detection, the output thereof becomes inevitably intermittent. It is also difficult to respond to periodic changes in the state of exhaust gas (exhaust timing of exhaust gas, flow amount of exhaust gas, concentration of PFC gas in exhaust gas, types of PFC gas in exhaust gas, and so forth). Therefore, even if controlled to reach predetermined recovery concentration, it is essentially impossible to stabilize recovery concentration. (See dotted line in FIG. 4.)
Also, the internal pressure of the first tank 3 is controlled at a constant level by taking in outside air in the conventional art mentioned above, so that the following problems (1) to (4) exist. (1) Exhaust gas is diluted as outside air is taken in, so that the recovery efficiency of PFC declines. (2) When the level of negative pressure of the first tank 3 becomes small (close to the atmospheric pressure) by taking in outside air, the control range of the internal pressure of the first tank 3 becomes small and it becomes difficult to respond to the flow rate fluctuations or the like of exhaust gas. As a result, it becomes difficult to control the internal pressure of the first tank 3 at a constant level. (3) Only when the internal pressure of the first tank 3 (output of the second pressure sensor 17) is constant, can pressure at the upstream side of the blower 2 (output of the first pressure sensor 14) be controlled at an accurate target level by controlling the blower 2 which uses the inverter 2a. However, as described above, it is difficult to set the internal pressure of the first tank 3 at a constant level due to a change in the state of exhaust gas. Thus, it becomes difficult to control the pressure on the upstream side of the blower 2 (output of the first pressure sensor 14) at an accurate target level. (4) In order to enlarge a control range of the internal pressure of the first tank 3, the level of negative pressure has to be large in advance so as not to reduce the level of negative pressure of the first tank 3 even when outside air is taken in. when the level of the negative pressure of the first tank 3 is large, the load of the compressor 4 that raises the pressure of a gas flow from the first tank 3 becomes high, Accordingly, the running cost of the compressor 4 increases.
The present invention was made considering these circumstances. Its object is to provide a perfluoro compound separation recovery method which can stabilize recovery concentration while sensitively responding to a change in the state of exhaust gas, and which can lower the costs of a separation recovery system, and a perfluoro compound separation recovery system therefor.
In order to achieve the object above, a first aspect of the present invention is a perfluoro compound separation recovery method which separates and recovers perfluoro compound gas by contact with a membrane which permeates more diluent gas than perfluoro compound gas, after the pressure of diluted mixed gas consisting of perfluoro compound gas and diluent gas is raised high. The method includes the steps of separating high-pressurized diluted mixed gas into a permeable gas flow rich in diluent gas and a non-permeable gas flow rich in perfluoro compound,gas by contact with the above-mentioned membrane; repeatedly separating the non-permeable gas flow into another permeable gas flow and another non-permeable gas flow by contact with another membrane having the same functions as those of the membrane mentioned above; detecting the pressure or flow rate of the other permeable gas flow; controlling the flow rate the other non-permeable gas flow so as to set the pressure or flow rate constant; and recovering the other non-permeable gas flow. A second aspect of the present invention is a perfluoro compound separation recovery system having a means to raise the pressure of diluted mixed gas consisting of perfluoro compound gas and diluent gas; and a membrane to separate high-pressurized diluted mixed gas into a permeable gas flow rich in diluent gas and a non-permeable gas flow rich in perfluoro compound gas. The system includes another membrane to separate the above-noted non-permeable gas flow into another permeable gas flow and another non-permeable gas flow; a means to detect the pressure or flow rate of the other permeable gas flow; a means to control the flow rate of the other non-permeable gas flow; and a means to recover the other non-permeable gasflow.
The present inventors repeated thorough studies on the perfluoro compound separation recovery method and the perfluoro compound separation recovery system therefor. As a result, by providing constant pressure or flow rate of the other permeable gas mentioned above (gas rich in diluent gas which is provided by repeatedly separating the non-permeable flow that was separated with an initial membrane and is rich in PFC gas, with another membrane), nearly no fluctuations were found in the concentration of PFC gas after treatment, even if the concentration of PFC gas was uneven in diluted mixed gas to be treated, thereby achieving the present invention.
In other words, the perfluoro compound separation recovery method of the present invention includes the steps of detecting the pressure or flow rate of the other permeable gas flow; and controlling the flow rate of the other non-permeable gas flow so as to make the pressure or flow rate constant. Thus, even if there are fluctuations in the concentration of PFC gas in diluted mixed gas, the concentration of PFC gas after treatment can be kept roughly constant.
When the perfluoro compound separation recovery method of the present invention includes the step of controlling the pressure of diluted mixed gas at a predetermined fixed level before increasing the pressure of diluted mixed gas by returning an appropriate amount of the permeable gas flow to diluted mixed gas, and when the control is carried out without taking in outside air in the step, diluted mixed gas is not diluted with outside air. Thus, the recovery efficiency of PFC improves. Since outside air is not taken in when the pressure of diluted mixed gas before the pressure increase is controlled constant as mentioned above, the control range of the pressure of diluted mixed gas before the pressure increase is not reduced, Therefore, even if the states of the diluted mixed gas change, the method can easily respond to the change. Moreover, as outside air is not taken in as mentioned above, the load can be reduced when the pressure of diluted mixed gas is increased.
Since the perfluoro compound separation recovery system of the present invention has a means to detect the pressure or flow rate of the other permeable gas flow, an expensive concentration analyzer is unnecessary and costs can be reduced. Furthermore, since pressure or flow rate can be continuously detected, the system can respond to the periodic changes of the state of exhaust gas, and the concentration of PFC gas after treatment can be kept roughly constant.